The invention relates to an electrically operable valve, and particularly, but not exclusively, to such valves for use in fuel injection systems for internal combustion engines.
In many fields of technology there exists the problem of supplying very small but variable quantities of fluid. For this purpose, electromagnetically operated valves of the type in which the cross-section of the orifice is constant, and in which the quantity of fluid passing through is determined by the length of time it is open, are generally used. In the case of fuel injection systems for internal combustion engines, for example, such valves are located in the fuel feed line or even constructed as injection valves. Electromagnetic valves, however, have a long response time so that they can only obey in a delayed manner the instructions of an electromagnetic control appliance, which determines the quantity injected in dependence upon engine parameters and environmental parameters. With sharply varying load states such as occur, for example, in motor vehicles, in particular, optimal running of the engine is not guaranteed.
Valves with considerably shorter switching times and correspondingly low inertia can be manufactured with the aid of piezoceramic or magnetostrictional devices, which are connected to the valve body and contract almost without any delay when current flows through them and thereby likewise produce an almost unretarded raising of the valve body from its seat. The switching time of such valves is approximately 0.05 to 0.1 ms in contrast to electromagnetically operated valves, in which the switching time is at least 1 ms. However valves incorporating magnetostrictional or piezoceramic devices have up to now not proved to be practical because their valve lift is only of the order of 20 .mu.m (micron). This means that variations in temperture, wear and manufacturing tolerances can influence the effective valve lift and with it, have a lasting influence on the quantity of fluid dispensed by the valve.